Agriculture Reference
In-Depth Information
establishment of a new equilibrium of forces within the tissue. Molecules involved
in regulation cell wall extensibility are likely to play a key role in this process.
Since the wall of any cell contributes to and is part of an extracellular matrix that
unites a number of cells throughout a tissue, morphogenesis in one region has the
potential to alter the pattern of stress in a region of tissue at a distance. Depending
on the biophysical character of the material, the variability of this character in
space and time, and the variability of the compressive forces generated by growth,
complex but entirely predictable changes in morphogenesis could result, as indicated
by various modelling approaches. The question remains, however, whether these
theoretical considerations reflect an endogenous mechanism. At the local cellular
level, evidence in animal tissue strongly supports biophysical components of a
signalling mechanism. In plants, the experimental evidence remains sporadic.
The plant cell wall is a complex composite material, and modelling of the phys-
ical characteristics of such a material is non-trivial. Moreover, the physical forces
predicted to play a role in linking morphogenic events across a tissue are likely to
be transduced via a number of cell walls associated with different cell layers. Again,
predicting, measuring and manipulating the physical characteristics of such a com-
plex material at cellular spatial resolution is not trivial. A key aspect of research
in this area in animal systems has been the development and application of novel
microtechniques to manipulate and measure the physical environment of cells in
culture. As yet, such techniques have not been applied to plant systems and, until
this happens, the role of biophysical parameters in intercellular communication in
plants is likely to remain debatable.
4.4.1 Connections between the cell wall and the cytosol as a conduit
for intercellular signalling
As mentioned in the previous section, advances in animal research with respect to
the significance of the ECM in intercellular signalling have been dependent on the
characterisation of the molecular bridges connecting the ECM and the cytoskeleton
(Hynes, 1987). A key component of these bridges are the integrin receptors that
(on the extracellular face) can interact with ECM proteins (such as fibronectin,
vitronectin and laminin) whereas on the cytosolic face integrins can be anchored to
various cytoskeletal-associated elements (such as vinculin and talin). These sites of
adhesion play a key role in the perception and transduction of information. Although
it is clear that plant cells must also contain some type of molecular bridge between the
ECM and the internal components of the cell, the identity of these bridges remains
essentially uncharacterised. Proteins that are immunologically cross-reactive with
antibodies raised against vitronectins are present in plant tissue extracts, but the
genes encoding such proteins have not yet been identified (Sanders
et al.
, 1991;
Wagner
et al.
, 1992; Zhu
et al.
, 1993). More recently, interest in this area has
focused on a novel family of WAKs.
WAKs consist of a cysteine-rich extracellular domain that has sequence simi-
larity to epidermal growth factor repeats, suggesting that they can interact with an
